This project studies how language works, how it develops in the child, and how it is computed by the brain. The key idea is that language is an interaction between words, which are pairings between a sound and a meaning stored in memory, and rules, which combine words into larger words and sentences. It uses irregular (bring-brought) and regular (walk-walked) inflection to study this interaction, because irregular forms are memorized, like words, whereas regular forms are generated by a rule ("add -ed"), like sentences, but they are matched in meaning, grammar, and complexity. The pacing of language development will be studied in twins: if a part of language develops in closer synchrony in identical twins (who share all their genes) than fraternal twins (who share half their genes), genes may affect the acquisition of that part; if the pattern is the same in the two kinds of twins, acquisition would be paced instead by environmental input. By measuring the development of vocabulary, word combinations, and past tense forms (especially errors like breaked, which could only be produced by rule), one can determine whether word-memory and rule-combination are differently influenced by biological maturation and by information in the environment. Processing of language in the brain will be studied using magnetoencephalography (MEG), the measurement of rapid magnetic signals from the cortex, which can map brain activation as a person assembles (regular) or retrieves (irregular) past tense forms. These studies will be complemented with event-related functional Magnetic Resonance Imaging of past tense generation, and by measuring lesion-symptom correlations in neurological patients asked to provide past tense forms for regular, irregular, and novel verbs. The benefits of documenting the development and neural bases of words and rules include better understanding of children with delayed and disordered language, and of the abilities of patients with lesions or degeneration of the brain.